Final answer:
When the Fed buys Treasury bonds, it increases the money supply by injecting money into the banking system.
Step-by-step explanation:
The action that is most likely to result in an increase in the money supply is when the Fed buys millions of dollars in Treasury bonds. When the Fed buys these bonds, it injects money into the banking system, increasing the reserves of commercial banks. As a result, banks have more money available to lend, which leads to an increase in the money supply.